As well known, the Doherty structure is a mainstream in current power amplifier (PA) design in base stations. High peak power and high efficiency in back-off power level makes the Doherty amplifier an ideal way for power saving.
Currently technology on PA design is using Doherty structure to meet linearity and efficiency requirement. As shown in FIG. 1, the conventional Doherty amplifier includes two amplifier stages, a main amplifier and a peak amplifier. The main amplifier is an amplifier that primarily amplifies the average power of the input signal. The peak amplifier is an amplifier that continuously amplifies the input signal when the main amplifier is saturated. The main amplifier is connected to the output through an impedance-inverter, usually a quarter wavelength transmission line or an equivalent lumped network. To compensate the quarter wavelength transmission line, a quarter wavelength is added to the input of peak amplifier.
The Doherty amplifier generally operates in two condition, small signal and high signal. During small signal period, only the main amplifier is active and the peak amplifier does not work. The impedance Zp in FIG. 1 is infinite in theory. Two microstrips ZmT and ZcT make the main amplifier load Zm much higher than 50 Ohm, and thus the main amplifier performs with high efficiency. During high signal period, the peak amplifier works and the main amplifier load Zm normally turns to 50 Ohm. The two amplifiers' peak power will combine.
Although Doherty structure can maintain high efficiency at back-off power level, the back-off Voltage Standing Wave Ratio (VSWR) of the main amplifier is determined by the hardware, such as the characteristic impedances of the two microstrips ZmT and ZcT, etc. Thus, in a conventional Doherty amplifier structure, the impedance with the best back-off efficiency is not on the VSWR circle of the main amplifier.
Since the distance (VSWR) between high power at 50 ohm and high efficiency at high impedance load of main transistor is defined by transistor itself, it is often difficult to find a suitable peak transistor to fit the distance. Therefore, it is difficult to get required VSWR to reach maximum efficiency point at high impedance of main transistor.